It is well known to those skilled in the art that gelled or crosslinked water-soluble polymers are useful in enhanced oil recovery and other oil field operations. They have been used to alter the permeability of underground formations in order to enhance the effectiveness of water flooding operations. Generally, polymers along with an appropriate crosslinking system are injected in an aqueous solution into the formation. The polymers then permeate into and gel in the regions having the highest water permeability.
Because of environmental concerns as well as cost for disposing of a produced brine which is defined as the brine co-produced with oil and gas, it is desirable to utilize the produced brine as the aqueous solution used for the polymers and appropriate crosslinking systems. Use of produced brines eliminates not only the cost associated with acquiring and pre-treating fresh water for use as the aqueous solution but also the disposal cost for the produced brine. Most produced brines are known to be hard brines, i.e., those having a divalent cation concentration greater than 1000 ppm. Chromium(III) carboxylates such as, for example, chromium acetate or chromium propionate, are the only known crosslinkers which can be used to produce stable gels in produced brines for near-wellbore treatment. See for example R. D. Sydansk, Acrylamide-Polymer/Chromium(III)-Carboxylate Gels for Near Wellbore Matrix Treatments, Proceedings SPE/DOE Seventh Symposium on Enhanced Oil Recovery (1990). Although a chromium(III) salt is not as toxic as a chromium(VI) salt, it is not an environmentally desirable compound and its use may require additional costs to assure the integrity of the injection wells to avoid contamination of ground water sources.
Therefore, a more environmentally suitable gelling composition that can form stable gels in produced brines for near-wellbore as well as in-depth treatments has been developed. However, the crosslinking agents are not inexpensive. In order to reduce cost in enhanced oil recovery, a process for reducing the permeability of oil-bearing formation by reducing the total quantity of crosslinking agents is highly desirable.